1. Technical Field
The disclosed principles of the invention are related generally to an internal combustion engine with turbocharger, and in particular to the use of waste heat or blowdown gases produced by the engine to drive the turbocharger, and also to the use of fuel as a working fluid to drive the turbocharger or other power producing means prior to introduction into the engine.
2. Description of the Related Art
Increasingly, automobiles are being manufactured with boost systems such as turbo charging or supercharging systems to improve engine efficiency. The various kinds of boost systems involve trade-offs in efficiency, reliability, and cost. The diagram of FIG. 1 shows selected elements of a turbocharged engine system 100 according to known art. The system comprises an internal combustion engine 102 that includes a plurality of cylinders 104, intake ports 108 extending between an intake manifold 106 and respective ones of the cylinders 104, and exhaust ports 110 extending between respective ones of the cylinders 104 and an exhaust manifold 112. An exhaust line 114 extends from the exhaust manifold 112 of the engine 102 to an intake of a first turbine 116. The output of the first turbine 116 is coupled via an exhaust line 114 to the intake of a second turbine 118, whose output is coupled to the downstream exhaust system 120, which typically includes a catalytic converter, a muffler, and a tailpipe, all of which are well known in the art, and thus not shown in detail. The illustrated system also comprises a bypass channel 121 extending between exhaust lines 114, and that includes a bypass valve 122, sometimes referred to as a wastegate. The first and second turbines 116, 118 are coupled via a driveshaft 124 to a compressor 126. The intake of the compressor 126 is coupled to a fresh air intake 128, and the output of the compressor 126 is coupled via a charge air line 134 to a cooler 130. A charge air line 134 extends between the cooler 130 and the intake manifold 106. Coolant is introduced via coolant input 132. In many systems, exhaust gas is reintroduced into the cylinders to modify the combustion characteristics of engines. In such cases, the gas will be introduced with the fresh air from downstream of turbines 116 and 118 to upstream of the compressor 126, or from upstream of turbines 116 and 118 to downstream of the compressor 126.
The operation of an internal combustion engine is well known in the art, and will therefore be described only in relevant part. As the piston in each cylinder descends during the intake stroke, charge air is drawn into the cylinders 104 via the intake ports 108, and, depending on the type and design of engine, fuel enters by one of a number of paths, such as by direct injection, port injection, carburetion, etc. The fuel and air mixture in each cylinder 104 is compressed by the respective piston in the compression stroke and caused to combust (in the case of conventional compression ignition-type engines, fuel is injected at or near the top of the compression stroke). Combustion of the fuel with oxygen from the charge air in the cylinder produces heated combustion gases creating elevated pressure within the cylinder, driving the piston, coupled to the engine crankshaft, downward, imparting rotational energy to the crankshaft.
At or near the bottom of the piston's travel, the exhaust valve opens, opening a path from the cylinder to the exhaust port and releasing residual gas pressure in the cylinder. As the crankshaft continues to rotate, the piston reciprocates in the cylinder 104, driving the remaining exhaust gases out through the respective exhaust port 110 to the exhaust manifold 112 during the exhaust stroke. The portion of the engine cycle after combustion, after the exhaust valve opens, and during which the piston is near the bottom of its stroke, is referred to herein as exhaust blowdown. The gas that exits the cylinder during this portion of the cycle does so in response to the significant pressure that remains in the cylinder once the piston has bottomed out. After exhaust blowdown, the remaining gases are expelled as the piston rises during the exhaust stroke.
In some high efficiency exhaust system designs used with naturally-aspirated engines, exhaust blowdown initiates a high-velocity stream of exhaust gases that creates a relative vacuum in the exhaust system, which assists in drawing the remaining exhaust from the cylinder. As the piston nears the top of the exhaust stroke, and the intake valve begins to open, the vacuum from the still exiting exhaust gases draws charge air and fuel into the piston via the intake valve, with the timing of the closure of the exhaust valve selected to prevent unburned fuel from escaping via the exhaust port. This process is sometimes referred to as exhaust scavenging, and permits the engine to evacuate virtually all the exhaust gases, including gases that occupy the unswept portion of the cylinder, thereby permitting a higher volume of combustibles to enter the cylinder. Exhaust scavenging generally requires specialized and individually tailored exhaust system components that enable the creation and support of the high-velocity gas stream, and is also generally limited to a narrow range of engine rpm's.
Even in naturally-aspirated engines that do not benefit from exhaust scavenging, there is only minimal resistance to gas flow in the exhaust system, so there is little resistance to the piston as it pushes the gases out. Once the exhaust gases escape past the valve, the exhaust pressure outside the piston drops to nearly ambient. However, in a turbocharged engine system such as that illustrated in FIG. 1, the turbines 116, 118, impede the exhaust flow, creating back pressure between the turbine 116 and the exhaust ports 110. This places a load on the engine, as a portion of the energy produced by the engine 102 is expended by the pistons driving the gases out against the back pressure in the exhaust system. As the still-pressurized exhaust gases enter the high-pressure turbine 116, the gases are allowed to expand, and the energy released is converted to rotational energy by the turbine, rotating the driveshaft 124. As the gas exits the high-pressure turbine 116, it may yet be only partially expanded, depending on the volume of the gases being produced by the engine 102. The gases pass from the high-pressure turbine 116 to the low-pressure turbine 118, where they are further expanded, imparting additional rotational energy to the driveshaft 124. From the low-pressure turbine 118, the exhaust gases pass into the downstream exhaust system 120 and are released to the atmosphere.
As engine speed increases, more exhaust gases are produced and the back pressure increases. While this causes the turbines to rotate faster, it also increases the load on the engine, and at some point the energy transfer efficiency of the high-pressure turbine begins to drop off. The bypass valve 122 is controlled to open as back pressure increases, venting a portion of the exhaust gas directly to the low-pressure turbine 118, which has a higher capacity than the high-pressure turbine 116, and can more efficiently extract energy from the increased volume of exhaust gas.
The energy generated by the expansion of the exhaust gas is transmitted by the driveshaft 124 to the charge air compressor 126, which draws in and compresses charge air from the charge air input 128, and transmits the compressed charge air to the cooler 130 via the air line 134. The cooler 130 transfers heat from the compressed air to a coolant, such as air passing through a radiator, or via a closed-loop cooling system to a remote radiator. The compressed air is cooled to increase its density and further increase its heat capacity, and is then moved to the intake manifold 106, and thence to the individual cylinders 104 via the intake ports 108. By introducing compressed charge air in the cylinders, the amount of oxygen in each cylinder is increased, which means that more fuel can also be added, increasing the power capacity of the engine. The higher heat capacity of the greater air mass helps control combustion temperature, which in turn assists in controlling the production of smog and pollution causing compounds. Additionally, the pressurized charge air entering the cylinders 104 imparts some energy to the piston during its intake stroke, which partially offsets the load exerted by the back pressure in the exhaust system. This is in comparison with a naturally-aspirated engine that draws charge air in by vacuum and that places a load on the engine during the intake stroke, but expends little energy driving exhaust gases out. While the energy imparted by the pressurized charge air against the piston does not fully offset the cost of driving the exhaust turbines, the additional power and efficiency gained by the higher combustion levels results in a significant net gain.
While the engine system 100 described with reference to FIG. 1 includes high- and low-pressure turbines and a bypass valve, there are many other known exhaust turbine configurations having one, two, or more turbines, with and without bypass means. For example, the following patents and patent application publication are directed to various aspects of efficient operation of a turbocharger, all of which are incorporated herein in their entirety: U.S. Pat. Nos. 4,930,315; 6,751,956; 2006/0042246.